Binary stars and star clusters

Binary stars

The types of binary stars are visual binaries, spectroscopic binaries, eclipsing binaries and astrometric binaries.

Visual binaries

Visual binary stars are two stars with a large enough separation that they can both be seen through a telescope or  a pair of binoculars. 5-10% of visible stars are visual binaries.

Sirius A and B viewed through an X-ray telescope: By NASA/SAO/CXC - CHANDRA X-ray Observatory CXC Operated for NASA by SAO, Original image, Public Domain, Wikimedia Commons

Spectroscopic binaries

Spectroscopic binaries are stars that are so close together that when viewed visually, they would appear as one star, or stars really close to each other. Scientists measure the wavelengths of the light the stars emit to determine their binary nature, based on their results.

Eclipsing binaries

Eclipsing binaries are two stars whose orbits are at an angle when viewed from the Earth. When one passes in front of the other, less light reaches Earth, so an eclipse occurs.

Algol B orbiting Algol A:
By Dr. Fabien Baron, Dept. of Astronomy, University of Michigan, Ann Arbor, MI 48109-1090, labels indicating phase added by User:Stigmatella aurantiaca - Derivative work of File:Algol AB movie imaged with the CHARA interferometer.gif with additional labels indicating phase, CC BY-SA 3.0, Wikimedia Commons

Astrometric binaries

Astrometric binary stars are stars that orbit each other. The primary star seems to be orbiting an unseen companion. The orbit is calculated based on the astrometric motion of the visible star. They are an important type of binary stars as they combine an object of very low luminosity.

Optical double stars

Optical double stars appear close together, although that is only a coincidence. They are actually far apart. Winneckle 4 is an example of this.

Winneckle 4.
By Reevesastronomy at English Wikipedia - Transferred from en.wikipedia to Commons by Thwblueboy using CommonsHelper., Public Domain, Wikimedia Commons

Mizar and Alcor are two naked eye double stars making up the end of the Big Dipper asterism handle. Alcor is the fainter of the two.

Mizar is actually quadruple star system, made up of two binary star systems. Although they appear close, Alcor and Mizar aren't gravitationally bound.

Star clusters

Stars in star clusters form in groups, and are gravitationally bound. Some stars will reach escape velocity from the protocluster and become runaway stars, while the rest will become part of the star cluster. They will orbit each other until they die.

Open clusters

Open clusters are groups of up to a few thousand stars that were formed from the same molecular cloud. They have roughly the same age. There have been more than 1100 open clusters already discovered in the Milky Way, and more are believed to exist.

Globular clusters

Globular clusters are densely packed collections of old stars. There are at least 150 globular clusters in the Milky Way, and they are at least 10 billion years old. They likely formed early, before the galaxy flattened into a spiral disc.

A globular cluster:
By NASA, The Hubble Heritage Team, STScI, AURA - Great Images in NASA Description, Public Domain, Wikimedia Commons

Some globular clusters, like M13 in the Hercules constellation, can be seen with the naked eye. They are mostly low-mass red stars and intermediate-mass yellow stars. The density of stars in globular clusters is more than the density of stars around the Sun. The amount of elements heavier than helium is 1-10% of those same elements found in the Sun.

When a cluster ages, the mass of the main sequence turnoff stars decreases. By finding the mass of the main sequence turnoff stars, the age of the cluster can be determined.

A zero-age main sequence star is the time when a star first joins the min sequence on the Hertzsprung-Russell diagram by burning hydrogen in its core through nuclear fusion.

Reflection

Today we learnt about different types of binary stars, and about visual double stars, which aren't visual binaries. We also learnt about star clusters, and how they "live" together peacefully. The lesson was interesting but slow. There were too many words in the slides too.